Abstract: A paradigmatic case of multi-band Mott physics including spin-orbit and
Hund's coupling is realised in Ca$_2$RuO$_4$. Progress in understanding the
nature of this Mott insulating phase has been impeded by the lack of knowledge
about the low-energy electronic structure. Here we provide -- using
angle-resolved photoemission electron spectroscopy -- the band structure of the
paramagnetic insulating phase of Ca$_2$RuO$_4$ and show how it features several
distinct energy scales. Comparison to a simple analysis of atomic multiplets
provides a quantitative estimate of the Hund's coupling $J=0.4$ eV.
Furthermore, the experimental spectra are in good agreement with electronic
structure calculations performed with Dynamical Mean-Field Theory. The crystal
field stabilisation of the d$_{xy}$ orbital due to $c$-axis contraction is
shown to be important in explaining the nature of the insulating state. It is
thus a combination of multiband physics, Coulomb interaction and Hund's
coupling that generates the Mott insulating state of Ca$_2$RuO$_4$. These
results underscore the importance of Hund's coupling in the ruthenates and
related multiband materials.